1. Field of the Invention
The subject invention relates to a cooling assembly for cooling an electronic device such as a microprocessor or a computer chip.
2. Description of the Prior Art
These electronic devices generate a high concentration of heat, typically a power density in the range of 5 to 35 W/cm2. Accordingly, research activities have focused on developing more efficient cooling assemblies capable of efficiently dissipating the heat generated from such electronic devices, while occupying a minimum of space.
A forced air cooling assembly typically includes a heat exchanger and a heat sink, and cools the electronic device by natural or forced convection cooling methods. The electronic device is attached to the heat sink and transfers heat thereto. The heat exchanger typically uses air to directly remove the heat from the heat sink. However, air has a relatively low heat capacity. Such forced air cooling assemblies are suitable for removing heat from relatively low power heat sources with a power density in the range of 5 to 15 W/cm2. However, the increased computing speeds have resulted in a corresponding increase in the power density of the electronic devices in the order of 20 to 35 W/cm2, thus requiring more effective cooling assemblies.
In response to the increased heat produced by the electronic devices, liquid-cooled cooling assemblies, commonly referred to as liquid cooled units (“LCUs”) were developed. The LCUs employ a heat sink in conjunction with a high heat capacity cooling fluid, like water or water-glycol solutions, to remove heat from these types of higher power density heat sources. One type of LCU circulates the cooling fluid through the heat sink to remove the heat absorbed from the heat source affixed thereto. The cooling fluid is then transferred to a remote location where the heat is easily dissipated into a flowing air stream with the use of a liquid-to-air heat exchanger and an air moving device such as a fan or a blower. These types of LCUs are characterized as indirect cooling units since they remove heat form the heat source indirectly by a secondary working fluid. Generally, a single-phase liquid first removes heat from the heat sink and then dissipates it into the air stream flowing through the remotely located liquid-to-air heat exchanger. Such LCUs are satisfactory for a moderate heat flux less than 35 to 45 W/cm2.
The amount of heat transferred between the heat sink and the cooling fluid is dependent upon a heat transfer coefficient therebetween. The heat transfer coefficient is dependent upon a temperature gradient between the heat sink and the cooing fluid, with the higher heat transfer coefficient corresponding to higher temperature gradients, i.e., the higher the temperature gradient between the heat sink and the cooling fluid, the more heat the cooling fluid will remove. The amount of heat stored in the base plate and each of the fins varies according to the distance from the heat source, with the highest concentration of heat occurring directly above the heat source in the base plate.
The U.S. Pat. No. 5,304,846 issued to Azer et. al., and the U.S. Pat. No. 6,422,307 issued to Bhatti et. al., each disclose a heat sink for a LCU. The heat sink assemblies include a base plate with a plurality of fins having smooth sidewalls extending upwardly from the base plate. In operation, the fins absorb heat through the base plate, with less heat being absorbed the farther the fin gets from the heat source. The cooling fluid is introduced into the heat sink at the outer periphery thereof, either parallel to or impinging on the fins. The flow of cooling fluid absorbs a portion of the heat from the outer periphery of the heat sink before contacting the highest heat concentration in the heat sink. The heat absorbed from the outer periphery of the heat sink increases the temperature of the cooling fluid, thereby lowering the temperature gradient between the cooling fluid and the heat sink when the cooling fluid finally arrives at the highest concentration of heat in the heat sink, thereby decreasing the efficiency of the heat sink.